This invention pertains to telephone switching systems in general, and to a phase-locked loop for use in distributed control digital telephone switching systems in particular.
The ITT 1210 is an advanced digital switching system manufactured in the United States by International Telephone and Telegraph Corporation. The ITT 1210, which was priorly identified as DSS-1, is described by C. G. Svala, "DSS-1, A Digital Local Switching System With Remote Line Switches", National Telecommunications Conference, 1977, pp. 39: 5-1 to 39: 5-7. As described by C. G. Svala, the ITT 1210 comprises three basic elements, i.e., line switches, a switching network, and a system control. A line switch interfaces with subscriber lines and to one or more PCM (pulse code modulation) lines. A line switch may be co-located with the switching network or may be located remoted therefrom. The switching network coupled to the line switches by the PCM lines comprises a number of digital switching modules called switch groups each of which includes a time-space-time network. The system control includes a pair of processors with associated program and data memories. The system control directs the operation of the switching network and the line switches. Communication of control information between the system control and a line switch is via a common channel superimposed on each PCM line. This common channel approach is described in U.S. Pat. No. 4,125,743, issued Nov. 14, 1978 to R. E. Steidl and assigned to a common assignee.
A large percentage of the equipment costs in a digital switching system is attributable to the line switches. Also the line switch design determines the transmission performance per subscriber line, traffic handling and line signaling functions of the switching system.
It is therefore desirable to provide an improved line switch having a low cost per subscriber line and improved transmission performance.